In such a conventional system for example, as shown in Japanese Patent application Laid-open No. 36906/89, a valve spring is used for biasing the engine valve in a valve-closing direction, and the engine valve is driven in a valve-opening direction by the driving member against a spring force of the valve spring. However, in the conventional system using such a spring force, since there is a limit in the natural frequency of the valve spring, the operation of the engine valve corresponding to a high rotation of the engine may become difficult.
In another prior art design, an air spring is used for biasing the engine valve in the valve losing direction as shown in FIG. 6. In this prior art design, a bottomed cylindrical lifter 35 driven by a cam 40 is slidably fitted in a guide bore 36 provided in a body 1 of an engine and is abutted against a rear or stem end of a valve shaft portion 5a of an intake valve 5 or an exhaust valve. A piston 37 fixed to a mid-point of the valve shaft portion 5a is slidably fitted in a sleeve 39 with a front surface of the piston 37 being exposed to an air pressure chamber 38.
In this prior art design, since the intake valve 5 or the exhaust valve is biased in the valve-closing direction by the air pressure of the air pressure chamber 38 and so the resonance limit inherent to the natural frequency need not be taken account of, it is possible to drive the engine at a higher speed. However, the piston 37 is formed in a flat plate shape, and in order to secure a stroke of the piston 37 accompanying the opening and closing operations of the intake valve 5 or the exhaust valve, it is necessary to make the axial length of the sleeve 39 relatively long.